w/RB: An Immense World: : How Animal Senses Reveal the World Around Us, Ed Yong

An Immense World: How Animal Senses Reveal the World Around Us, Ed Yong, 2022.

Overall, a good book. Yong writes well, and sometimes has very nice turns of phrase, though I’d say his gift is more for clarity and content than lyricism. The downsides of the book — small ones but nevertheless there — is that he often doesn’t go as deeply into the mechanisms and neurophysiology of sensing as I would like. It is also the case that one gets a bit of whiplash from looking first at this organism, and then at that, and then at that — but I don’t see how that could have been avoided in this sort of book.

To summarize briefly and incompletely, here are some of the points I found most interesting:

  • What we think of as a single sense (e.g., vision) can be quite complex. All of the following can be separate: distinguishing light from dark areas; color vision (and bi- tri- and tetra-achromatism); ability to see polarized and/or UC and/or infrared light; and more.
  • Also, the same sense can be configured and deployed in different ways: the shape of an organism’s visual field is tightly bound with its role in the food web; an organism may have one, two or multiple eyes, and may be able to move them independently; and so on.
  • Some senses seem easy to evolve, in that they have been independently evolved at many different points in time. And then lost, and then re-evolved.

April 2023 – February 2024

Introduction

The book begins with a fanciful description of a room with different creatures in it, including a human, a robin, an elephant, a spider, and so on. It uses this to make the point that the different creatures, although all in the same room, have radically different impressions of the room and its occupants. What is evident to one is invisible to another. An organism’s very particular view of its environment – is referred to as its umveldt, coined by Jacob Uexkull in 1909.

Yong comments that what he is interested in is animals as animals:

Animals are not just stand-ins for humans or fodder for brainstorming sessions. They have worth in themselves. We’ll explore their senses to better understand their lives. “They move finished and complete, gifted with extensions of the senses we have lost or never attained, living by voices we shall never hear,” wrote the American naturalist Henry Beston. “They are not brethren, they are not underlings; they are other nations, caught with ourselves in the net of life and time, fellow prisoners of the splendour and travail of the earth.”

—Ed Yong, An Immense World, p 7

The introduction introduces a number of themes:

  • The appearance of a particular sense may cause the organism to evolve capacities that take advantage of it, such as vision laying the ground for planning and strategic thinking
  • An umveldt is constrained, but the constraints keep the organisms from being overwhelmed with input: “the poverty of this environment is needful for the certainty of action, and certainty is more important than riches.” [Jacob Uexkull]
  • It is difficult to say how many senses there are. It can both be difficult to divide some of them into distinct categories (is vemerol nasal sensing part of olfaction), and is the platypus’s bill, which contains cells that respond to pressure and electric fields a multi sensory organ, or does it have a single electro-touch sense?
  • Thomas Nagel’s “What is it like to be a bat?” Attempting to understand an organism’s umveldt requires a leap of imagination.

The Umwelt concept can feel constrictive because it implies that every creature is trapped within the house of its senses. But to me, the idea is wonderfully expansive. It tells us that all is not as it seems and that everything we experience is but a filtered version of everything that we could experience. It reminds us that there is light in darkness, noise in silence, richness in nothingness. It hints at flickers of the unfamiliar in the familiar, of the extraordinary in the everyday, of magnificence in mundanity.
[…]
Listen to treehoppers, and you realize that plants are thrumming with silent vibrational songs. Watch a dog on a walk, and you see that cities are crisscrossed with skeins of scent that carry the biographies and histories of their residents. Watch a swimming seal, and you understand that water is full of tracks and trails. 

ibid., p 14-15

C1: Leaking Sacks of Chemicals: Smells and Tastes

  • This chapter is about olfaction. It begins with dogs. Humans and dogs have the same olfactory machinery, but dogs have more of everything. Also, dogs olfactory apparatus is partitioned from their breathing apparatus so that smell is a continuous thing to them, rather than strobbing with inhalations and exhalations. In fact, when a dog exhales, the air exits through slits in the sides of its nostrils which creates rotating vortices of air that waft odorants into the nostril.
  • “Smells diffuse and sweep, flood and swirl.” An olfactory sensorium is “a shimmering environment where nothing has a hard boundary:” there are focal areas, but everything is seeping together. Also smells linger, so they provide information about both the present and past.
  • Humans excel at discriminating among odors — they are much better than mice, for example.
  • The Jahai people of Malaysia use a dozen words (compared to three in English) to characterize odors.
  • Chemicals are the most ancient and universal source of sensory information.

Unless you actually stuck your nose over some benzaldehyde, you couldn’t guess that it smells like almonds. If you saw dimethyl sulfide drawn on a page, you couldn’t foresee that it carries the scent of the sea. Even similar molecules can produce immensely different smells. Heptanol, with a backbone of seven carbon atoms, smells green and leafy. Add another carbon atom to the chain and you get octanol, which smells more like citrus. Carvone exists in two forms that contain exactly the same atoms but are mirror images of each other: One smells of caraway seeds and the other of spearmint. Mixtures are even more confusing. When mixed, some pairs of odors still smell distinct, while others produce a third smell that’s unlike the two parents.

– ibid., footnote, p 27
  • There aren’t enough different receptors to account for the range of odors we can detect, so odor coding must involve combinations of receptors.
  • Olfaction also has individual differences. To some people with the gene OR7D4, body odor smells like vanilla.
  • Be suspicious of claims that attempt to pit one animal against another: it is very unclear what it means to say one organism’s sense of smell is better: can it detect more odors, detect fainter scents, exhibit finer discrimination among different smells?
  • It is unclear if human pheromones exist. (A pheromone is a chemical used for intra-species signaling)
  • Ants have many pheromones. Light ones that waft into the air are useful for emergency signals which require rapid spread and response; medium weight pheromones that dissipate more slowly are useful for marking trails, that will eventually vanish when they are no longer relevant; heavy weight pheromones (cuticular hydrocarbons) are found on an ants body and are useful for signaling identity and role. Queens can use the latter substances for stopping a worker from breeding (I thought they were infertile), or marking a worker for punishment.
  • Ants and odorant receptor genes: Fruit flys have 60, honey bees 140, most ants 300-400, and clonal raider ants have 500.
  • Elephants have 2000 odorant receptor genes, and their extended mobile trunk enables them to sample odors in sophisticated ways.
  • Some scientists believe the main purpose of animal olfaction is not to detect chemicals but to support navigation through the world.
  • Example of oil company that adds ethyl mercaptan to the oil in its pipelines, and the watching for gatherings of vultures to detect pipeline leaks.
  • Marine birds and other marine animals respond to the scent of dimethyl sulfide to locate food. DMS is chemical released by plankton when it is eaten by krill. It is not very water soluble and rises into the air, creating what humans detect as a sort of ‘seaweedy’ smell. DMS is associated with geological features like seamounts and depth that effect the nutrients in the water and thus the amount of plankton and krill, and this ultimately produces a sort of olfactory topography over the ocean that seabirds can use to find food.
  • Many animals smell in stereo — using a pair of nostrils. A particularly notable example of this are the forked tongues of snakes, which while not nostrils act to convey odorants to the vomeronasal organ in its mouth. The flicks of a snake’s tongue are engineered to gather odors: the make 10-20 arcs a second, and create minuscule vortices of air that draw in odorants from the left and right side of the snake.
  • The vomeronasal organ

Many backboned animals have two distinct systems for detecting odors. The main one includes all the structures, receptors, and neurons that I described in the head of a dog at the start of this chapter. The vomeronasal organ is its sidekick; it has its own kinds of odor-sensing cells, its own sensory neurons, and its own connections to the brain. It’s usually found inside the nasal cavity, just above the roof of the mouth. Don’t bother trying to feel around for yours, though. For some reason, humans lost our vomeronasal organ during our evolution, as did other apes, along with whales, birds, crocodiles, and some bats.

ibid., 46
  • Smell versus taste. Smell works at a distance; taste requires contact. Smell has huge individual differences (the US Military was unable to develop a stink bomb because it could find an odor that was offensive to all people); taste is much more universal, and has essentially five components.
  • Taste. Catfish can taste with their whole bodies, and can also detect the different between l- and d- amino acids. (It turns out that certain marine worms and clams can flip l-amino acids into their d-forms.
  • GPRCs (G-Protein-Coupled Receptors)

At the start of this chapter, we saw that dogs and other animals detect smells using proteins called odorant receptors. These are part of a much larger group of proteins called G-protein-coupled receptors, or GPCRs. Ignore the convoluted name; it doesn’t matter. What matters is that they are chemical sensors. They sit on the surface of cells, grabbing specific molecules that float past. Through their actions, cells can detect and react to the substances around them. This process is temporary: After the GPCRs are done, they either release or destroy the molecules that they’ve grabbed. But one group of them bucks this trend: opsins. They are special because they keep hold of their target molecules, and because those molecules absorb light. This is the entire basis of vision. This is how all animals see using light-sensitive proteins that are actually modified chemical sensors.

In a way, we see by smelling light.

ibid., p 52

…break in reading…

C2: Endless Ways of Seeing: Light

  • Jumping spiders: 4 pairs of eyes on each quadrant of head; each eye has a lens in front and a retina in back. The eyes can move independently, but both members of a pair can also both focus on a single object of interest. The front-facing eyes have color vision, a good visual acuity and a narrow field of vision; the eyes on either side are immobile but have a much larger field of view, and allow the spider to use its front eyes to track moving objects.
  • Eyes:
    (1) In the animal kingdom eyes come in a vast array of forms. They can come in pairs, eights or hundreds; they can be monfocal or bifocal; they can have lenses made of protein or rock; they can be located on heads, mouths, arms or armour.
    (2) While eyes and photocells vary drammatically from creature to creature, they all detect light using proteins called opsins in conjunction with a chromophore usually derived from Vitamin A.
  • Evolution of Eyes
    Eyes evolve through four phases of increasing complexity (cf. Dan-Eric Nilsson):
    (1) simple photoreceptors that detect the presence or absence of light
    (2) directionality via pigmented ‘shades’ that enable assessment of the direction of a light source.
    (3) groups of shaded photoreceptors that create a 2-D montage of incoming light
    (4) High resolution vision emplolying devices like lenses to sharpen the images
    Once you have stage 4, organisms can interact (at a distance) in all sorts of ways. It underlies predation, defensive camoflage, conflict and courtship. One hypothesis is that stage 4 vision was one of the drivers of the Cambrian explosion.
  • What has not changed in eye evolution: opsin
    With the exception of opsin proteins, which evolved only once and have never changed, eyes have evolved multiple times. For example, the jellyfish, with stage 2 eyes, has evolved them at least nine times.
  • Zebras do not have stripes to camoflage themselves from predators; predators, especially given that they are nocturnal hunters, do not have visual acuity to resolve the stripes. Instead, zebra stripes appear to protect them fomr tsetse flies, by foiling their ability to land.
  • Humans outshine almost every other animal at resolving detail (raptors are the exception).
  • Scallops have dozens to hundreds of eyes, all neon blue and arranged along the edges of their shells. Each eye is located at the end of a tentacle, and has a pupil that directs light to a mirror made of square crystals of guanine that redirects it to a retina. In between their eyes are tentacles that are used for chemical sensing. One hypothesis is that scallops use chemical sensing to detect predators, and their eyes to detect food. Perhaps for a scallop, smell is the fine-grained detailed sense, and vision is more like human touch.
  • Some animals like sea stars and sea urchins (as well as certain microbes) appear to use their entire body as an eye. They have photoreceptors all over, and the pigmentaion (or other features) of their body allows them to sense the direction light is coming from.
  • Birds — an interesting thing is that the location of their blind spots vary.
    In spite of great acuity to their left and right, vultures (and raptors?) have large blind spots above and below their heads, and, tilting their heads down as they fly means there is a blind spot directly ahead of them. This is why they can crash into tall narrow objects like wind turbines.
    In contrast, herons have 180° vertical vision without moving their heads so that they can see fish at their feet;
    and mallards have completely panoramic vision.
  • Animals that live in flat habitats (cows, kangaroos, rabbits, fiddler crabs, water striders) have visual fields that wrap almost all the way around their heads, and their acute zones are horizontal stripes that give them a view of the entire horizon at once.
  • Killer flies have ultrafast vision. That is, in vision it typically takes photoreceptors time to react to incoming photons and for nerve signals to travel to the brain. Killer flies can do this and react within 6 to 9 milliseconds; in contrast, it takes human photoreceptors between 30 and 60 milliseconds to register a photon. Another way to thing of this is a measure of temporal resolution called the Critical Fusion Frequency (CFF), the rate at which individual images blend together: in humans its about 60 Hz; for most flies its about 350 Hz, and probably higher for killer flies.
  • The tapetum is a reflective layer that bounces incoming light that has missed the retina back onto the retina. This enables them to see in lower light, and is responsible for the chatoyance in dogs, cats and deer and other mammals. The tapetum of the reindeer gets larger during the lower light period of the winter, and coincidentally changes the color of their eyes from gold to blue.
  • Giant Squid have massive eyes because big eyes are excel at detecting large glowing objects at a distance — specifically the sperm whale which is their only predator.

Summary. With the exception of opsin, the protein that interacts with chromophores, eyes have evolved multiple times and in multiple ways. Their number, size, acuity, ability to see color, ability to detect varying intensities of light, the shapes of their visual fields, the degree of independence of movement, and location on the body are incredibly variable. Even within seemingly closely related organisms, visual fields and other features for detect predators or prey are adapted to an organisms environment and lifestyle. 

reading break…

C3: Rurple, Gruple, Yurple: Color

  • Opsins. Color vision depends on a family of proteins called “opsins” that respond to particular wavelengths of light. Most humans are trichromatic, which means they have three types of opsin that respond to red, green and blue wavelengths. As we shall see, different organisms — even different mammals — may have different opsins and thus different (or no) color vision.
  • Opponency. When a particular color is perceived, it is detected by all the opsins, but they produce differing responses depending on how close the color being perceived is to the color they maximally respond to: Thus red light will stimulate the long opsin the most, but will also stimulate the green opsin a bit, and the blue opsin a little. Color vision works by the comparison of responses from the different type of opsins: the means one can see a continuous spectrum of colors
  • Monochromates. Some organisms lack color vision, either because they only have rods, or they have only one type of cone (which given the necessity of opponency fo color vision is as good as having no cones at all.
  • Advantages of color vision. Most tasks necessary for survival do not require color vision, as shown by the many organisms that lack it. That raises the question of why it evolved at all? Physiologist Vadim Maximov suggests that color vision evolved 500 Ma during the Cambrian period, when most life existed in shallow seas. These seas would have been filled with flickering rays of light, which would cause the brightness of a particular area to change by several orders of magnitude over seconds: this would have been enormously confusing to monochromatic eyes which can only detected brightness. Chromatic vision, with its opponency, would have fared better in these circumstances because the chromatic visual system is detecting relative proportions of wavelengths: under these circumstance color would be much more robust and stable than brightness.
  • Monochromats, dichromats, and so on.
    – Monochromates can detect about 100 shades of gray.
    – Dichromates can detect about 100 steps from yellow to blue, which multiples with the grays to produce on the order of 10,000 colores
    – trichromats add another hundred steps from red to green which elevates perception into millions of shades. However, more is not necessarily better: for some tasks, such as detecting borders and edges which may facilitate, in the case of primates, locating insects that resemble sticks or stems in a noisy visual environment.
  • UV Perception. Quite a few organisms — insects, reptiles, fish and birds — have UV receptors; this also includes many though not all mammals. Humans do not see UV because our lenses filter it out, but if we lose our lens (as painter Claude Monet did) we can see UV as a whitish-blue (which is why Monet’s late paintings contain white-blue water lillies).
  • Spectral colors: We can see some colors that do not map to the wavelenth spectrum. One example is purple, which we ‘see’ when the red and blue cones are equally stimulated. I’ve been told that pink another such color. ‘
  • Tetrachromats — such as birds, reptiles, insect, and freshwater fish — ought to be able to see 100x more colors than we can.
  • The larval zebrafish eye has three types of vision: black and white in the part that looks up (and is customized for detecting aerial preditotors), UV detection in the parts that look straight ahead (and that allow it to detect UV-absorbing plankton), and tetrachromatic vision in the part that scans the horizon and the area below the fish.
  • Mantis shrimp have an incredibly diverse set of light receptors, includign the ability to detect circularly polarized light.
  • Degrees of Freedom. Vision is an incredibly diverse sense, when looked at a cross organisms. Organisms differ in the opsins they have and thus the wavelengths they can detect, what they do inresponse to that detection (opponency or non-opponency), the number and position of their eyes, the degree of mobility of their eyes, and so on.
  • Reef fish are yellow and blue because that color scheme camoflages them to fish and other predators; it is only because humans are very good a discriminating between yellow and blue that they appear so colorful to us.
  • Very often, the abiility to see color in particular ways appears before it is useful. Genes for opsins get doubled, and then evolve to be tuned to different wavelengths. Over time, flowers, etc., evolve to take advantage of this acidentally enhanced discrimination.

Guide by evolution, eyes are living paintbrushes. Flowers, frogs, fish, feathers and fruit all show that sight effects what is seen, and that much of what we we find beautiful in nature has been shaped by the vision of our fellow animals. Beauty is not only in the eye of the beholder. It arises because of that eye.

Ed Yong, The Immense World, p 115

reading break…

C4: Pain: The Unwanted Sense

  • Naked mole rats. Can survive long periods in high CO2 environments, which would normally cause pain in mucus membranes due to build up of carbonic acid. But NMRs appear insensitive to acids, capsicin, and many — but not all substances (e.g. mustard) — that cause pain in many organisms. They also recoil from pinches and burns. The implied claim is that their sense of pain has been customized to the characteristics of the niche they inhabit.
  • Nociceptors, a term coined by Charles Sherrington in the early 1900’s, are a class of neurons that are involved in sensing stimuli that (may) cause pain: intense heat, cold, pressure and toxins and acids and chemicals released by inflammation. Nociceptors vary in size, sensitivity, and transmission rates.
  • The grasshopper mouse which preys on scorpions is insensitive to scopion venom — in fact when its nociceptors detect the venom they stop firing, making the venom into a pain-killer.
  • Nociception verssu pain. The claim is that nociception is the sense, and that pain is the (possibly) conscious experience produced in the cortex as a result of nocioception.
  • Pain and consciousness and robots. There is a lack of consensus regarding whether pain requires consciousness, and what sort of reactions, if any, would be evidence of pain. Some investigators consider long-term behavioral change in the wake of nociception to be evidence of pain; others disagree. It is noted that robots can be programmed to respond to stimuli in all the ways that would be taken to signify pain in an organism.
  • Trout experiment. In a now-classic experiment trout were injected in the lips with acetic acid, and showed long-term behavioral changes (rocking, ignoring unfamiliar objects that they would have normally been wary of) that can be interpreted as indicating pain.
  • Squid vs. Octopi. Started from a common ancestor, but have been distinct for 300 million years. Squid are open-ocean organisms and cannot touch every part of their body; octopi inhabit reefs and ocean-floor structures, make dens in which they can hide, and can touch every part of their bodies. When hurt, squid seem to feel pain (or at least turn up their nociception) over their entire bodies; octopuses act as though they have localized pain.
  • Insects behave as if they do not feel pain — walking on crushed limbs, mating or eating while being consumed by a mate or wasp larva. But even there, it is difficult to say whether they do not feel pain, or they simply have a much higher tolerance.
  • What is necessary to experience pain? A cortex? A critical number of neurons? A sufficient degree of neuronal interconnectivity? We don’t know, and it’s hard to tell how we could reach any sort of consensus.

reading break…

C5: So Cool: Heat

  • Hibernation involves lowering body temperature and slow metabolism. Hibernating animals incur sleep debt; in fact they must come out of hibernation to catch up on sleep.
  • TRP channels. Animals use a variety of temperature sensors: the most-studied are TRP channels. There are multiple TRP sensors that can be tuned to different tempertatures, and the tunings vary between animals depending on their lifestyle.
  • Variation among organisms. Animals have wide variations in the temperatures under which they can function: Sahara silver ants can tolerate temperatures up to 127° F as can the Pompeii worm that lives near undersea vents; snow flies are active at 21° F and ice worms spend their entire lives on glacial ice.
  • Heat => IR. We sense heat because the vibration of molecules that is heat produces electromagnetic radiation in the infrared spectrum. If we represent the visible and infrared spectrum as the length of a human arm, the visible part occupies the wide of a hair.
  • Parasites and warm-blooded animals. Parasites can sense the constant temperature of warm-blooded animals as a signal, and move in that direction. DEET and citronella appear to function not by disrupting the sense of smell (at least in ticks), but by blocking their ability to sense temperature.
  • Pit Vipers. The pits of pit vipers (rattlesnakes, cottonmouths, copperheads, moccasins) may function somewhat like eyes with a narrow pupil-like opening that enables directionality, and a retina-like membrane that responds to IR; the pits also come in pairs, but nothing was said about whether that permits binocularity. But they do not have particularly good resolution, having only thousands of sensors.
  • Temperature control as camouflage. When ground squirrels encounter rattlesnakes (but not when they encounter harmless gopher snakes that can’t sense infrared) the puff up their tails and direct blood into them, making them appear larger in the infrared.

reading break…

A Pause for Reflection on the book

I’m finding the book interesting, but… I’m a bit dissatisfied. One thing I miss is that it often says little or nothing about the physiology or neurophysiology. Sometimes it does a bit — as when discussing vision and the role opsin — but mostly not. I also find the way the book is breaking up chapters to be a bit artificial. Are “contact and flow” really a separate sense (or senses) from “vibration” and that from “hearing?” One might make a case that the experience or umweldt might be really different, but mostly that is difficult to get at in the case of animals; and even if one were able to do that, it seems that, as noted early, the sort of unweldts captured under the rubric of “vision” are going to be radically different. So I think I’m feeling a bit dissatsified because of this lack of rigor, and that it seeming more like a book with a very large set of ‘isn’t this cool’ accounts. Not that that’s bad, per se, but I’d hoped for more.

C5: So Cool: Heat

  • Hibernation. Hibernation is such an intense state of inactivity that ground squirrels have to emerge from it to sleep.
  • TRP Channels. Animals use a variety of sensors; the best studies are TRP Channels. These consist of groups of proteins that respond to particular temperatures (and intensity of temperatures).
  • Temperature adaptations range from Sahara Silver Ants and Pompeii worms which can survive at temperatures in the 120° range, and snow flies and glacier worms that spend the entire lives below freezing, and will die if held in a human hand.
  • Fire Beetles:

Arriving at a fire, the beetles have perhaps the most dramatic sex in nestled the animal kingdom, mating as a forest burns around them. Later, the females lay their eggs on charred, cooled bark. When the wood-eating grubs hatch, they find an Eden. The trees they devour are too injured to defend against insect larvae feeding within them. The predators that might eat them are put off by the smoke and heat emitted from the embers and ashes. In peace, they thrive, mature, and eventually Ay of in search of their own blazes.

Ed Yong, The Immense World, p. 142

Incomplete section

C6: A Rough Sense: Contact and Flow

  • Sea Otters. As a small mustelid it needs to eat a lot to maintain its temperature in the ocean; this in turn requires that it have very specialized sensory apparatus for feeding, and in this case that sensory apparatus involves a very fine sense of touch. They can discriminate between textures that differ in less the a quarter millimeter in spacing, and they can identify textures 30x faster than a human.
  • Humans themselves have a very discriminating sense of touch. We have different cells that respond to continuous pressure (Merkel cells), tension and stretch in the skin (Ruffini cells), and fast and slow vibrations (Pacinian and Meissner corpuscles). Collectively they produce the sensation of touch.
  • The star nosed mole has a ring of `11 paired appendages around its snout, the 11th of which is smaller but far more sensitive than the others. Its appendages touch-and-lift whatever it encounters about a dozen times a second, and assess pressure and vibration as they do so. It can detect and consume prey in as little as 120 milliseconds, with an average of 230 ms (about the smallest interval of time a human can discriminate).
  • Birds. Birds have touch sensors in their beaks, although their nature and distribution depends on the lifestyle of the bird species. Some birds can essentially use sonar — their beak thrust creates pressure waves that are distorted by distant objects, and they can sense those distortions.
  • The Emerald Jeweled Wasp has a stinger that includes sensors that allow it to locate a particular part of a cockroaches brain, which it stings to immobilize it.
  • Whiskers, bristles and feathers. There are many examples of long structures that are used to stimulate sensory cells, such as whiskers and feathers. Some mammals can move their whiskers back and forth (whisking) several times a second as they move.
  • Manatees have a prehensile oral disk between their mouths and nostrils that are packed with sensory cells. They also have vibrasse, sensory hairs, all over their body that allow them to sense the flow of water due to current, other manatees, or other animals.
  • Harbour Seals have about a hundred whiskers on their snouts and faces, and these enable them to track fish by the hydrodynamic wakes they leave while swimming. Hydrodynamic wakes can persist for minutes, and a harbor seal can track a herring from almost 200 yards away. Following a wake is similar to the way other animals follow smell trails — they move in and out of it and figure out the gradient. The whiskers themselves have an undulating surface that repeatedly widens and narrows, and this reduces the vortices left by the whiskers themselves.
  • Fish have a line of sensory cells on their bodies called the lateral line, which allow them to sense distortions in the flow of water around them. This allows them to detect objects, and other fish, and do things like swim in a coordinated way as a school, and to avoid collisions when the school is fleeing a predator.
  • Blind ‘cave’ catfish. Blind catfish found in a cave that has frequent torrential floods have evolved stiffer touch sensors based on their teeth, which grow out all over their bodies. The theory is that more typical sensor are overwhelmed by the flooding torrent, which favored the evolution of stiffer sensors.
  • Alligators have sensory bumps that are so acute that they can detect a single drop of water falling into their tank. They use the sensory bumps to scan the thin layer where air and water meet, so that they can detect prey. There is much speculation about other ways the sensors can be put to use in intraspecies communication, feeding, and raising young.
  • Peacocks. Peacocks have crest-like feathers on their heads that have a resonant frequency of 26 Hz, exactly the speed at which a mating Peacock shakes those feathers. These feathers have a smaller rudimentary feather at their base — called a filoplume — which may act as a mechanorecptor.
  • Filoplumes are found in many birds, and the speculation is that it allows them to monitor the position of their other feathers.
  • Bats’ wings are covered with small hairs that react only when air is moving from back to front, as it would when the bat is about to stall while flying.
  • Tiger Spiders and trichobothria.
  • Crickets and filiform hairs.

The filiform hairs of crickets and the trichobothria of spiders are almost inconceivably sensitive. They can be deflected by a fraction of the energy in a single photon – the smallest possible quantity of visible light. These hairs are a hundred times more sensitive than any visual receptor that exists, or could possibly exist. Indeed, the amount of energy needed to shift a cricket’s hairs is very close to thermal noise the kinetic energy of jiggling molecules. Put another way, it would be almost impossible to make these hairs more sensitive without breaking the laws of physics.

So why doesn’t everything in the world set them off? Why aren’t spiders constantly leaping at imagined insects, or crickets constantly fleeing from phantom spiders? Partly, the hairs only respond to biologically meaningful frequencies the kind produced by predators or prey. [and partly they only respond to signals from multiple hairs.]

Ed Yong, The Immense World, p 186-187

reading break…

C7: The Rippling Ground: Surface Vibrations

  • Surface Vibrations differ from sound waves in that they travel only in one direction along a planar surface — moving up and down relative to their direction of travel — unlike sound waves which spread out in all directions in three dimension. Because of this, small organisms that create surface waves can emit a wide variety of complex sounds because the sound does not need so much energy to propagate. In contrast, to be heard insects must focus their sounds in a narrow frequency range.
  • Tadpoles in the egg. In-egg tadpoles, a few days after being laid but before they hatch, can sense vibrations (e.g., those made when snakes are preying on egg masses), and hatch early so that they might escape.
  • Treehoppers.
  • Sand Scorpions (with slit sensilla).
  • Larva of ant lions.
  • Golden Mole. Not actually a mole, but acts kind of in the same way. It has a large bone — part of what is our inner ear apparatus, that is used to sense surface waves.
  • Elephant. Elephants can pick up subsurface vibrations, and will shift their posture to be more effective. Elephants also have local signals (e.g. of alarm) that are not recognized by elephants from distant herds.
  • Nephila spiders. In addition to the dynamic ways in which these spiders use their webs, there is another species of spider that will essentially ‘hack’ nephilia webs to hijack their prey.
  • Spider webs. It is argued that webs are actually a component of the spiders sensory apparatus, and can be seen as supporting a sort of distributed cognition. A hungry spider will tighten its web so as to more easily detect smaller prey; a well fed spider, if placed on a hungry spiders web, will act as though it is hungry.
  • The role of posture. Many organisms that sense the world via surface waves alter their posture to either improve or adjust their sensory capacity.

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C8: All Ears – Sound

  • => 10/20 (5 days post-Amsterdam)

C9: A Silent World Shouts Back – Echoes

  • => 10/20 (5 days post-Amsterdam)

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C10: Living Batteries — Electric Fields

Electrolocation is an active sense, in that the organism must do something — generate an electrical field – to use it. Electric fields are both omnidirectional and relatively local. With respect to its local nature, it resembles touch… and the organism must move if it wishes to get a larger ‘image’ of its environment. For instance, electric fishes will shimmy or wrap their bodies around objects they are curious about.

Electric organs consist of stacks of electrocytes — these evolved from muscle cells.

Electric fields are also instantaneous — electric fish can turn their fields on and off, and can encode information in the frequencies of pulsation. They are sensitive pulsations of a millionth of a second.

Organisms that use electric fields, and how they use them.

  • Strongly electric fish: Electric catfish (90 volts), torpedo rays, electric eels (up to 860 volts)
    — stun or kill prey & self defense; there is also an allusion to eels being able to remote control their prey’s bodies)
  • Weakly electric fish: elephant fishes and knife fishes
    — for communication and navigation: they court, claim territory and settle fights
  • Passive electroreception. All aquatic organisms produce weak electric fields by virtue of being alive and some organisms, such as sharks and sawfish, can detect those fields
    – they use them to locate prey (and in some cases, mates).
  • Guiana dophins
    – ???
  • Platypus and echidnas
    — location of (insectile) prey
  • Bees — Bees build up positive charges as they fly, and when they land on flowers negatively charged pollen grains can be attracted to the bee. Furthermore, flowers are surrounded by their own electric fields which have characteristics shapes that bees can detect.
    – flower ID and pollen uptake.
  • Spiders. Spider silk is negatively charged as it leaves the spiders body, and so it can be repelled by plants on which the spider is sitting, and used to ‘balloon’ the spider aloft.
    – movement.

Evolutionarily, the electric sense comes and goes. Vertebrates have lost it on at least four separate occasions; it is also frequently re-gained the ability. Perhaps it’s not difficult to evolve electroreceptors — after all, most senses translate a sensory impulse into electrical signals, but with electroreception no transduction is necessary.

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C11: They Know the Way — Magnetic Fields

We know little about magnetic fields sensing — we know that many organisms (some fish, some birds, some reptiles, some insects, etc.) can sense magnetic fields, but we do not know what the sensory apparatus is or where it is located in the organism.

Magnetic fields are incredibly weak; that said, they permeate almost everything, and they provide surficial coordinates because they have both inclination (which corresponds to latitude) and intensity — together these give most points on the earth’s surface a unique identifier.

Magnetic fields sensing enables

  1. migration to a particular place;
  2. navigation within a territory (home waters of fish/turtles; North Atlantic gyre for turtles);
  3. navigation back to birthplace;
  4. wayfinding underground in a local area (mole rat burrows) — but elsewhere in the chapter it is suggested that such sensing only works over large distances.

Organisms that can sense magnetic fields and how they use them:

  • Bugongs (moths) – navigate 600 miles to alpine caves to avoid summer heat (Australia)
  • Monarch butterflies – navigate thousands of miles from North to Central America
  • Many birds – long migrations; also exhibit Zugunrhe (migration anxiety). Thrush nightingales eat more and put on weight when they sense that they are approaching the Sahara, which promises a long taxing flight
  • cardinal fish – navigate back to reef of birth
  • mole rats navigate through their underground nests
  • harks and rays; mole ra
  • whales – more run aground during solar storms that disrupt the magnetosphere
  • sea turtles – Stay within the Atlantic Gyre when young; return to place of birth; seem to have a rich map of their home waters

Hypothesized mechanisms for magnetic field sensing

  • Magnetic ‘needles’ — some bacteria can grow chains of magnetite crystals. These could be tugged by magnetic fields, and could be tethered to molecules that would respond to direction and degree of stress.
  • Electromagnetic Induction. Organisms like sharks and rays that produced their own electric fields could potentially sense the distortions of those fields by the magnetic field as they move… This could also take place in birds — they have inner ears filled with conducting fluids which could in theory support electromagnetic induction.
  • Radical pairs. Pairs of molecules that are sharing an electron can be effected by magnetic fields, and offer a potential sensory mechanism. This was proposed by Klaus Schulten, and since the cryptochromin-flavin pairs have been proposed as candidates for the radical pairs.

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C12: Every Window at Once — Uniting the Senses

  • Mosquitoes use multiple strategies — they are attracted by heat, but only if they first detect CO2. Adese Egyptie originally preyed on a variety of animals, but at some point specialized in humans.

Imagine, instead, what it might be like to be a mosquito. Flying through a thick soup of tropical air, your antennae slice through plumes of odorants until they catch a whiff of carbon dioxide. Enticed, you turn into the plume, zigzagging when you lose track of it, and surging ahead whenever you pick it up. You spot a dark silhouette and fly over to investigate. You enter into a cloud of lactic acid, ammonia, and sulcatone molecules released by human skin. Finally, the clincher: an alluring burst of heat. You land, and your feet pick up an explosion of salt, lipids, and other tastes. Your senses, working together, have once again found a human.

— Ed Yong, An Immense World, p. 322
  • Multiple senses used to triangulate. No species uses one sense to the exclusion of all others. They combine them. And sometimes sense fuse, as in synesthesia. The antennae of ants detect both smell and touch — it is interesting to ponder whether these are treated separately or whether they fuse into some single hybrid sense.
  • Exafferance/Reafferance. Senses that are affected by movement require input from the organisms motor system to distangle sensations produced by the actions of external actors (exafferance) versus sensation produced by the organisms own actions (reafferance). Thus, bats need to ‘ignore’ the sounds they produce, so they can make correct inferences from the echoes.
  • Corollary discharge. A common solution across species is that the motor system will send a copy of its ‘commands’ to the perceptual system, so that those actions can be accounted for in computing what is perceived. This is called “corollary discharge.” This is why you can’t tickle yourself.
  • Distributed sensing. The case of octopuses. A sucker on a detached arm will attach to a fish or a bit wall, but not to another arm from the same octopus. It appears that octopus arms are, on their own, capable of very complex grasping and twisting and navigational behavior; the octopuses brain can control the arms, but there appears not to be much fine grained control; it is more that the brain serves to coordinate all the arms to work together, but without ‘micromanaging.’

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C13: Save the Quiet, Preserve the Dark — Threatened Sensescapes

  • Makes the point that sound and light and chemical pollution have huge ramifications for organisms with different umweldts. Light disrupts bird navigation, and sound disrupts bat navigation. “Sensory pollution is the pollution of disconnection.“
  • 83% of people — 99% of Americans and Europeans – are living under light polluted skies. Light at night is uniquely anthropogenic: that has never occurred under natural conditions. Street lights impact plant pollination by attracting moths that would otherwise pollinate plants. Red light decreases the impact, but it isn’t a panacea. There is no light that is good for everything, but blue and white is the worst,.
  • Noise pollution shrinks the perceptual world of organisms that depend on sound. It affects matting, predation, and no doubt other things as well.
  • There was a nice experiment done where sounds of a healthy coral reef were played where there was a dead one, and it attracted reef organisms that began to repopulate the site., Unfortunately this solution does not scale well.

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